Gas-phase polymerization is an economical process for the polymerization of olefins, such as ethylene and propylene, and the copolymerization of ethylene and/or propylene with C4-C8 alpha-olefins. Such gas-phase polymerization processes can, in particular, be designed as gas-phase fluidized-bed processes in which the polymer particles are kept in suspension by means of a suitable gas stream. Processes of this type are described in, for example, EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826. Other background references include EP 0 801 081 A, EP 2 743 279 A, and CN 102 161 715.
In such processes, the polymer particles produced in the fluidized bed are discharged continuously or discontinuously from the reactor and conveyed pneumatically to a product recovery system. The polymer particles inevitably contain small amounts of unreacted monomer as well as heavier hydrocarbons added to, or produced in, the polymerization process. For example, the polymer particles may contain saturated homologues of the feed monomers produced by hydrogen supplied to the reactor to control the molecular product of the polymer and/or condensable liquids, such as C4 to C6 alkanes, added to assist in heat removal. Thus, the product recovery system includes a degassing or purging vessel where unreacted monomers and heavier hydrocarbons are stripped from the polymer particles normally by countercurrent contact with an inert gas, typically nitrogen. The resulting inert gas stream, diluted with unreacted monomer and heavier hydrocarbons is recovered from the purge vessel and, after separation of the hydrocarbon components, may be partially recycled as the conveying gas or, in another embodiment, as part of the purge stream. Part of the effluent from the purge vessel is removed from the system and currently, since the concentration of unreacted monomer in this stream is too low to render its recovery economically feasible, the vent stream is flared or used as fuel. This not only represents a significant loss of valuable monomer, but also results in regulated environmental emissions. There is therefore a need for an improved product purge system for gas phase olefin polymerization processes in which the loss of unreacted monomers in the product vent stream is reduced or eliminated.